1. Field
The present invention relates to semiconductor device electronics in the areas of semiconductor-based logic, magnetoresistive devices, and magnetic sensor technology. The present invention includes a spin resonant tunnel diode and spin transistor that operate by controlling spin-polarized current flow using low applied voltages and/or magnetic fields (for magnetic field sensing only), and which are fabricated using traditional III-V semiconductors (i.e. no magnetic materials). These devices represent an improvement over conventional semiconductor device technology.
2. Description of the Related Art
The prospect of developing semiconductor electronic devices that exploit electron spin has motivated a broad research effort into the spin-related properties of semiconductor materials. Semiconductor spintronics has been identified as an emerging research direction for logic applications due to possible improvements in power consumption, which represents a fundamental limitation in scaling for silicon-based CMOS technologies beyond the next decade. Spin-based devices may also be applied to other semiconductor technologies (e.g. memory, optoelectronics, and quantum computation) with the possibility for enhanced performance and functionality (e.g. nonvolatility, high-speed, and high scalability).
Several spintronic device concepts have been proposed; however, the majority of these rely on magnetic metals or magnetic semiconductors (e.g. diluted magnetic or paramagnetic semiconductors). However, devices relying only on nonmagnetic materials are more attractive for practical application because they are more easily integrated into traditional device architectures, and they avoid the complex fabrication issues associated with the incorporation of magnetic materials (e.g. low magnetic solubility, low-temperature growth, conductivity mismatch, interface quality). Devices that avoid magnetic semiconductor materials also offer greater promise for operation at or above room temperature. Furthermore, devices in which the electron spin state is controlled using applied electric fields (rather than external magnetic fields) are favorable because electric fields may be modulated at high rates. Additionally, stray field effects are much less problematic for device operation when only electric fields are used.
An effective approach in developing spin-based semiconductor devices that do not require magnetic materials is to exploit the spin-orbit interaction in traditional III-V semiconductors, by which an electron with a non-zero momentum will experience an electric field also as an effective magnetic field (or pseudomagnetic field). The electric field causing this pseudomagnetic field may originate from a variety of sources, for example, (i) internal electric fields associated with the polar bonds in III-V semiconductors (bulk inversion asymmetry, BIA); or (ii) extrinsic electric fields introduced through asymmetric layer growth, differences in interface potential on two sides of a semiconductor quantum well layer, or the application of an electric bias to a gate above the semiconductor (structural inversion asymmetry, SIA, also known as the Rashba effect). Each of these sources of electric fields contributes to the total pseudomagnetic field experienced by the electrons. In a spin-based device, this pseudomagnetic field may be used to manipulate electron spin dynamically, through the application of a controllable electric field using a gate. One may also design a spin-based device that exploits the energy splitting between electron spin states parallel and antiparallel in relation to the pseudomagnetic field to control spin transport, for example, in a resonant tunnel diode geometry.
In applying the pseudomagnetic field generated by the spin-orbit interaction in a spin-based semiconductor device application, one must take into account the dependence of the pseudomagnetic field orientation on the electron momentum. This pseudomagnetic field structure (which differs in cases (i) and (ii) above) may lead to severe, undesirable design constraints for spin-based semiconductor devices. For the Rashba effect (case (ii)), the Rashba pseudomagnetic field (Rashba field) is oriented in the plane of the heterostructure with a direction that varies with electron momentum. In this case, the Rashba field will cause relaxation of an initially spin-polarized ensemble of electrons by inducing precession, as individual electrons with different momentum directions will experience a different precession axis. This causes a serious trade-off between the size of the Rashba field and the spin relaxation time. Spin manipulation in a practical device requires a strong Rashba field, but a strong Rashba field leads to an extremely short spin relaxation time. Thus, the time interval for spin manipulation is unmanageably short (100 fs-1 ps) when using a strong Rashba field. One may limit device operation to electrons undergoing ballistic transport, in which only a single electron momentum direction is involved, and thereby one relevant pseudomagnetic field orientation; however, this approach leads to considerable fabrication and material challenges as spin manipulation must occur within the electron mean free path.
The Rashba effect may be used for spin manipulation in the ballistic transport regime, as in a proposal proffered by Datta and Das in which spin injection and detection are achieved using ferromagnetic metal contacts (S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)). Also, the Rashba effect may be used in a spin resonant tunnel diode, as in a proposal proffered by Ting et al. that relies on the energy splitting between electron spin states induced by the Rashba pseudomagnetic field (D. Z. Y. Ting and X. Cartoixa, Appl. Phys. Lett. 81, 4198 (2002)). Unfortunately, these devices suffer from the undesirable constraints discussed above.
Accordingly, there is a need for nonmagnetic devices that exploit electric field control over the electron spin for device function without: (i) suffering from a trade off between spin control efficiency and spin relaxation; and (ii) being constrained to ballistic device geometries.